What describes a severe case of dandruff characterized by excessive waxy or greasy scales?

The severe case of dandruff characterized by excessive waxy or greasy scales is best described as pityriasis steatoides. This condition involves an overproduction of oil and leads to the accumulation of thick, greasy scales on the scalp and other areas of the body.

Pityriasis, in a broader sense, refers to various conditions that cause flaking of the skin, but it does not specifically emphasize the greasiness or oiliness associated with steatoides. Seborrheic dermatitis is often similar in presentation because it also involves flaky, oily skin, but pityriasis steatoides specifically highlights that excessive waxy quality. Psoriasis is a different skin disorder marked by red patches covered with thick, silvery scales and isn't primarily associated with greasy appearances. Therefore, pityriasis steatoides is the term that accurately captures the severity and the specific characteristics described in the question.